This study investigated the startup behavior of Oscillating Heat Pipes (OHPs) by comparing numerical simulation and on-orbit experimental data. Previous studies suggested that initial vapor-liquid distribution affects startup behavior. However, they provided no experimental evidence to validate this hypothesis because experimentally reproducing and specifying initial vapor-liquid distribution in OHPs is virtually impossible. Thus, a numerical approach is necessary to generate the initial vapor-liquid distribution and to understand the internal thermofluid behavior of OHPs. In this study, a one-dimensional numerical model of an OHP with check valves was first developed. Then, the model was compared with data from an on-orbit experiment. Finally, simulation of OHP startup behavior with several types of initial vapor-liquid distributions showed that OHP startup difficulty is due to localization of liquids in the cooling section. (C) 2017 Elsevier Ltd. All rights reserved.